The following will describe a conventional transformer for a switching power supply and a switching power supply using the transformer. FIG. 7 is a circuit diagram showing the switching power supply. In FIG. 7, reference numeral 1 denotes a power supply transformer and reference numeral 2 denotes a primary winding which is wound around a core 3 of the power supply transformer 1 ten times. Reference numeral 4 denotes a secondary winding which is wound outside the primary winding 2.
Reference numeral 5 denotes a 60V direct-current power supply which is connected to a terminal 2a of the primary winding 2. The other terminal 2b is connected to the drain terminal of a transistor 6, and the source terminal of the transistor 6 is connected to the ground.
Reference numeral 7 denotes an input terminal to which an oscillation signal of 40 kHz is inputted. The input terminal 7 is connected to the gate of the transistor 6.
A start terminal 4a of the secondary winding 4 is connected to the ground and aground terminal 8. An intermediate terminal 4b of the secondary winding 4 is connected to the anode side of a diode 9, and the cathode side of the diode 9 is connected to a 12V terminal 10. The cathode side of the diode 9 is connected to the ground via a capacitor 11.
The intermediate terminal 4b of the secondary winding 4 is connected to an end terminal 4c via a 0.5-turn (½ turn) winding 4d. The end terminal 4c is connected to the anode side of a diode 12, and the cathode side of the diode 12 is connected to a 15V terminal 13. The cathode side of the diode 12 is connected to the ground via a capacitor 14.
In this configuration, a wire is wound twice between the start terminal 4a and the intermediate terminal 4b of the secondary winding 4 and a wire 4d is wound 0.5 times between the intermediate terminal 4b and the end terminal 4c. 
The following will discuss the operations of the switching power supply configured thus. The primary winding 2 of the power supply transformer 1 has a wire wound ten times and a voltage of 60 V is applied to the primary winding. A power supply applied to the primary winding 2 is turned on/off by the transistor 6 at a frequency of 40 KHz. In other words, the power supply is converted into a high frequency power supply of 40 KHz.
Thus, due to electromagnetic induction, a pulse voltage of 12 V is induced between the start terminal 4a and the intermediate terminal 4b of the secondary winding 4. The wire is wound twice between the start terminal 4a and the intermediate terminal 4b. Then, the voltage is half-wave rectified by the diode 9, smoothed by the capacitor 11, and outputted as a 12V direct-current power supply to the 12V terminal 10.
Further, a pulse voltage of 3 V is induced between the intermediate terminal 4b and the end terminal 4c of the secondary winding 4. That is, a pulse voltage of 15 V is induced between the start terminal 4a and the end terminal 4c. Similarly, the voltage is half-wave rectified by the diode 12, smoothed by the capacitor 14, and outputted as a 15V direct-current power supply to the 15V terminal 13.
The power supply transformer 1 will be discussed below. FIG. 8 is a sectional view showing the power supply transformer 1 from the front. In FIG. 8, reference numeral 3 denotes the core formed of soft iron. The core 3 is composed of an inner core 3a and outer cores 3b and 3c which are connected in a ring to the inner core 3a and formed symmetrically with respect to the inner core 3a. 
First, the primary winding 2 is wound around the inner core 3a, and the terminals 2a and 2b are drawn (FIG. 7) from both ends of the primary winding 2. Further, the secondary winding 4 is wound outside the primary winding 2 via an insulating layer 15. The secondary winding 4 is first wound twice between the start terminal 4a and the intermediate terminal 4b, and the 0.5-turn winding 4d is wound between the intermediate terminal 4b and the end terminal 4c. 
As shown in FIG. 8, the 0.5-turn winding 4d passes between the inner core 3a and the outer core 3b from the front to the back. In other words, as shown in FIG. 9, the winding 4d passes only between the inner core 3a and the outer core 3b but does not pass between the inner core 3a and the outer core 3c. The 0.5-turn winding 4d is formed in this manner.
For example, as information on prior art documents relating to the invention of this application, patent document (Japanese Patent Laid-Open No. 2000-134926) is known.